Touch sensor and operating method thereof

ABSTRACT

A touch sensor includes: a pulse signal generator for generating a pulse signal of which pulse width is calibrated in response to a control code; a pulse signal transmitter for transmitting the pulse signal when a touch object is out of contact with a touch pad and stopping transmitting the pulse signal when the touch object is in contact with the touch pad; a pulse signal detector for detecting the pulse signal transmitted through the pulse signal transmitter; and a controller recognizing a non-contact state and adjusting the control code to calibrate the pulse width of the pulse signal when the pulse signal detector detects the pulse signal. In the above-described configuration, the contact of the touch object with the touch pad can be sensed more precisely, and the occurrence of a malfunction in the touch sensor due to changed operating conditions can be prevented.

The present invention relates to U.S. Pat. No. 7,605,805 which wasapplied by the same applicant.

TECHNICAL FIELD Background Art

The present invention relates to a touch sensor, and more particularly,to a touch sensor capable of sensing whether or not a touch object is incontact with the touch sensor using an electrostatic capacitance of thetouch object.

Korean Patent Application No. 2005-23382 discloses a touch sensor asshown in FIG. 1, which senses whether or not a touch object is incontact with the touch sensor by varying a difference in delay timebetween a touch signal and a reference signal using the electrostaticcapacitance of the touch object.

Referring to FIG. 1, the touch sensor includes a reference signalgenerator 10, a first signal generator 21, a second signal generator 22,a touch signal generator 30, and a low pass filter (LPF) 40.Specifically, the reference signal generator 10 generates a referencesignal ref_sig. The first signal generator 21, which includes a resistorR11 and a capacitor CAP, delays the reference signal ref_sig by aconstant delay time irrespective of whether or not the touch object isin contact with the touch sensor, and generates a first signal sig1. Thesecond signal generator 22, which includes a resistor R12 and a touchpad PAD, delays the reference signal ref_sig by a variable delay timeaccording to the electrostatic capacitance of the touch object andgenerates a second signal sig2. The touch signal generator 30, whichincludes a D-flip-flop, latches the second signal sig2 in response tothe first signal sig1 and generates a touch signal con_sig. The LPF 40filters the touch signal con_sig and outputs a filtered signal.

The touch signal generator 30 generates a touch signal con_sig having afirst level when the touch object is brought into contact with the touchpad PAD and the second signal sig2 has a longer delay time than thefirst signal sig1. On the other hand, the touch signal generator 30generates a touch signal con_sig having a second level when the touchobject is out of contact with the touch pad PAD and the second signalsig2 has a shorter delay time than the first signal sig1.

As described above, the touch sensor of FIG. 1 varies a difference indelay time between the first signal sig1 and the second signal sig2depending on whether or not the touch object is in contact with thetouch pad PAD.

However, when the touch pad PAD has poor touch sensitivity or the touchobject has very small electrostatic capacitance, a difference in delaytime between the first signal sig1 and the second signal sig2 cannot besufficiently varied, so that a malfunction may occur in the touchsensor.

Furthermore, the impedance of a circuit device included in each of thefirst and second signal generators 21 and 22 and a delay differencebetween the first and second signals sig1 and sig2 may vary withoperating conditions of the touch sensor, such as an operating powersupply voltage and the temperature and humidity of the atmosphere.

However, although the impedance of the circuit device included in eachof the first and second signal generators 21 and 22 is changed accordingto the operating conditions, the conventional touch sensor provides nocalibration element. As a result, the operating characteristics of thetouch sensor are varied according to the operating conditions and, whatis worse, a malfunction may occur in the touch sensor.

DISCLOSURE Technical Problem

The present invention is directed to a touch sensor and a method ofoperating the same in which the contact of a touch object with the touchsensor is precisely sensed.

Also, the present invention is directed to a touch sensor and a methodof operating the same in which the occurrence of a malfunction due tochanged operating conditions may be prevented.

Furthermore, the present invention is directed to a capacitancemeasurement circuit capable of reducing the influence of a noise.

Technical Solution

One aspect of the present invention provides a touch sensor including: apulse signal generator for generating a pulse signal of which pulsewidth is calibrated in response to a control code; a pulse signaltransmitter for transmitting the pulse signal when a touch object is outof contact with a touch pad and stopping transmitting the pulse signalwhen the touch object is in contact with the touch pad; a pulse signaldetector for detecting the pulse signal transmitted through the pulsesignal transmitter; and a controller for recognizing a non-contact stateand adjusting the control code to calibrate the pulse width of the pulsesignal when the pulse signal detector detects the pulse signal.

In an embodiment of the present invention, the pulse signal transmittermay include: a resistor; and the touch pad charged or discharged withthe pulse signal according to a resistance of the resistor and anelectrostatic capacitance of the touch object to inhibit thetransmission of the pulse signal.

In another embodiment, the pulse signal transmitter may include: avariable resistor of which a resistance varies with the control code;and the touch pad charged or discharged with the pulse signal accordingto the varied resistance of the variable resistor and the electrostaticcapacitance of the touch object to inhibit the transmission of the pulsesignal when the touch object is in contact with the touch sensor.

In an embodiment of the present invention, the pulse signal generatormay include: a clock signal generator for generating a clock signal; anda counter of which a counting value is set according to the control codeand counting by the counting value in response to the clock signal tovary the pulse width of the pulse signal.

In another embodiment, the pulse signal generator may include: a clocksignal generator for generating a clock signal; a signal delay unit forvarying the delay time of the clock signal according to the controlcode; an inverter for inverting an output signal of the signal delayunit; and a logic gate for performing a logic AND operation on the clocksignal and an output signal of the inverter to generate the pulse signalhaving a pulse width corresponding to the delay time of the clocksignal.

Another aspect of the present invention provides a method of operating atouch sensor.

The method includes: generating a pulse signal having a predeterminedpulse width; transmitting the pulse signal when a touch object is out ofcontact with a touch pad and stopping transmitting the pulse signal whenthe touch object is in contact with the touch pad; recognizing anon-contact state when the pulse signal is transmitted and recognizing acontact state when the pulse signal is not transmitted; and calibratingthe pulse width of the pulse signal in the non-contact state.

In an embodiment of the present invention, calibrating the pulse widthof the pulse signal in the non-contact state may include: obtaining acritical pulse width at which the pulse signal is not transmitted bygradually decreasing the pulse width of the pulse signal from themaximum value; obtaining a calibrated pulse width by adding a marginpulse width to the critical pulse width when a difference between thecurrent critical pulse width and a critical pulse width obtained in theprevious calibration operation is within the permitted limit; andcalibrating the pulse width of the pulse signal to the calibrated pulsewidth.

In another embodiment of the present invention, calibrating the pulsewidth of the pulse signal in the non-contact state may include:obtaining a critical pulse width at which the pulse signal is nottransmitted by gradually decreasing the pulse width of the pulse signalfrom the sum of a pulse width obtained in the previous calibrationoperation and the permitted limit; obtaining a calibrated pulse width byadding a margin pulse width to the critical pulse width when adifference between the current critical pulse width and a critical pulsewidth obtained in the previous calibration operation is within thepermitted limit; and calibrating the pulse width of the pulse signal tothe calibrated pulse width.

In yet another embodiment of the present invention, calibrating thepulse width of the pulse signal in the non-contact state may include:obtaining a critical pulse width at which the pulse signal is nottransmitted by increasing and decreasing the pulse width of the pulsesignal using a successive approximation method; obtaining a calibratedpulse width by adding a margin pulse width to the critical pulse widthwhen a difference between the current critical pulse width and acritical pulse width obtained in the previous calibration operation iswithin the permitted limit; and calibrating the pulse width of the pulsesignal to the calibrated pulse width.

Advantageous Effects

As described above, a touch sensor is capable of confirming if a touchobject is in contact with a touch pad depending on whether a pulsesignal is transmitted or not, so that the touch sensor can perform atouch sensing operation more precisely. Also, the pulse width of thepulse signal is periodically adjusted to operating conditions, thuspreventing the occurrence of a malfunction in the touch sensor due tochanging operating conditions. As a result, the operating reliability ofthe touch sensor can be enhanced.

DESCRIPTION OF DRAWINGS

FIG. 1 is a detailed circuit diagram of a conventional touch sensor;

FIG. 2 is a block diagram of a touch sensor according to an exemplaryembodiment of the present invention;

FIG. 3 is a detailed circuit diagram of a touch sensor according to anexemplary embodiment of the present invention;

FIG. 4 is a detailed circuit diagram of a touch sensor according toanother exemplary embodiment of the present invention;

FIG. 5 shows a correlation between the delay time of a signal delay unitof FIG. 4 and the pulse width of a pulse signal;

FIG. 6 is a detailed circuit diagram of a touch sensor according to yetanother exemplary embodiment of the present invention;

FIG. 7 is a detailed circuit diagram of a signal delay unit SIGDaccording to an exemplary embodiment of the present invention;

FIG. 8 is a circuit diagram of a pulse signal transmitter according toanother exemplary embodiment of the present invention;

FIG. 9 is a flowchart illustrating a method of operating a touch sensoraccording to an exemplary embodiment of the present invention;

FIG. 10 is a flowchart illustrating a calibration operation (step S10)of FIG. 9 according to an exemplary embodiment of the present invention;

FIG. 11 is a flowchart illustrating a calibration operation (step S10)of FIG. 9 according to another exemplary embodiment of the presentinvention;

FIG. 12 is a flowchart illustrating a calibration operation (step S10)of FIG. 9 according to yet another exemplary embodiment of the presentinvention;

FIG. 13 is a graph illustrating a method of finding a critical pulsewidth in the calibration operation of FIG. 12;

FIG. 14 is a circuit diagram of a capacitance measurement circuitconfigured to sense a contact state according to an exemplary embodimentof the present invention;

FIG. 15 is a circuit diagram of the capacitance measurement circuitincluding a delay time calculator/data generator of FIG. 14;

FIGS. 16 through 18 are diagrams illustrating operations of thecapacitance measurement circuit of FIG. 15;

FIG. 19 is a circuit diagram of a capacitance measurement circuitaccording to another exemplary embodiment of the present invention; and

FIG. 20 is a circuit diagram of a toggle circuit of a T-flip-flop ofFIG. 19.

MODE FOR INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail. However, the present invention is not limited tothe exemplary embodiments disclosed below, but can be implemented invarious types. Therefore, the present exemplary embodiments are providedfor complete disclosure of the present invention and to fully inform thescope of the present invention to those ordinarily skilled in the art.

FIG. 2 is a block diagram of a touch sensor according to an exemplaryembodiment of the present invention.

Referring to FIG. 2, the touch sensor may include a pulse signalgenerator 1, a pulse signal transmitter 2, a pulse signal detector 3,and a controller 4.

Specifically, the pulse signal generator 1 receives a code value of acontrol code “code” from the controller 4, sets the pulse width of apulse signal “pul” according to the code value of the control code“code”, and generates the pulse signal “pul” with the set pulse width.

The pulse transmitter 2 includes a touch pad PAD in which a touch objecthaving a predetermined electrostatic capacitance contacts. The pulsetransmitter 2 directly transmits the pulse signal “pul” to the pulsesignal detector 3 when the touch object is out of contact with the touchpad PAD, while the pulse transmitter 2 transmits the pulse signal “pul”not to the pulse signal detector 3 but to the touch pad PAD when thetouch object is in contact with the touch pad PAD.

In this case, the touch object may be any object having a predeterminedelectrostatic capacitance, for example, a human body in which a lot ofcharges may be accumulated.

The pulse signal detector 3 receives the pulse signal “pul” from thepulse signal transmitter 2, detects the pulse signal “pul”, andtransmits the detection result to the controller 4.

The controller 4 generates an output signal “out” based on the detectionresult of the pulse signal detector 3 and outputs the output signal“out” to an external apparatus, so that the external apparatus can beinformed of whether the touch object is in contact with the touch padPAD or not. Also, the controller 4 periodically performs a calibrationoperation such that the pulse width of the pulse signal “pul” isadjustable to the current operating conditions in a non-contact state.

In FIG. 2, the impedance of a circuit device included in each of thepulse signal generator 1 and the pulse signal transmitter 2 of the touchsensor and the touch sensitivity of the touch pad PAD may vary withoperating conditions, such as an operating power supply voltage and thetemperature and humidity of the atmosphere. Thus, the range of the pulsewidth in which the pulse signal detector 3 can detect the pulse signal“pul” transmitted by the pulse signal transmitter 2 also varies with theoperating conditions of the touch sensor.

Therefore, the controller 4 of the present invention varies the pulsewidth of the pulse signal according to the operating conditions so thatthe pulse signal detector 3 can always precisely detect the pulse signal“pul” transmitted by the pulse signal transmitter 2, thus preventing theoccurrence of a malfunction in the touch sensor due to variableoperating conditions.

FIG. 3 is a detailed circuit diagram of a touch sensor according to anexemplary embodiment of the present invention.

Referring to FIG. 3, the pulse signal generator 1 may include a clocksignal generator GEN and a settable down counter SDC, the pulse signaltransmitter 2 includes a resistor R and a touch pad PAD, and the pulsesignal detector 3 is embodied by a T-flip-flop TFF.

The clock signal generator GEN generates a clock signal “clk” andtransmits the clock signal “clk” to the settable down counter SDC.

The settable down counter SDC generates a pulse signal “pul” of whichpulse width varies according to a code value of a control code “code”transmitted from the controller 4. Specifically, the settable downcounter SDC of which a count value is set according to the code value ofthe control code “code”, leads the pulse signal “pul” to make an upward(downward) transition at the start of a counting operation, and leadsthe pulse signal “pul” to make a downward (upward) transition at the endof the counting operation, so that the pulse width of the pulse signal“pul” may vary with the code value of the control code “code”.

The resistor R has a predetermined resistance and obtains theelectrostatic capacitance of a touch object that is in contact with thetouch pad PAD. Thus, when the touch object is in contact with the touchpad PAD, the resistor R and the touch pad PAD are charged or dischargedwith the pulse signal “pul” according to the resistance of the resistorR and the electrostatic capacitance of the touch object, and thetransmission of the pulse signal “pul” to the T-flip-flop TFF isinhibited. On the other hand, when the touch object is out of contactwith the touch pad PAD, the resistor R and the touch pad PAD are neithercharged nor discharged with the pulse signal “pul”, and the pulse signal“pul” is transmitted to the T-flip-flop TFF.

When receiving the pulse signal “pul”, the T-flip-flop TFF issynchronized with a rising edge or falling edge of the pulse signal“pul” and toggles an output signal. When receiving no pulse signal“pul”, the T-flip-flop TFF does not toggle the output signal.

When the T-flip-flop TFF outputs the toggled output signal, thecontroller 4 externally outputs an output signal “out” for informing auser of non-contact of the touch object with the touch pad PAD. When theT-flip-flop TFF does not output the toggled output signal, thecontroller 4 externally outputs an output signal “out” for informing theuser of contact of touch object with the touch pad PAD.

As described above, the touch sensor of FIG. 3 allows or inhibits thetransmission of the pulse signal “pul” depending on whether the touchobject contacts the touch pad PAD, so that a user can easily andprecisely confirm the contact or non-contact of the touch object withthe touch pad PAD.

FIG. 4 is a detailed circuit diagram of a touch sensor according toanother exemplary embodiment of the present invention.

Referring to FIG. 4, a pulse signal transmitter 2, a pulse signaldetector 3, and a controller 4 are respectively the same as those ofFIG. 3, but a pulse signal generator 1′ includes a clock signalgenerator GEN, a signal delay unit SIGD, an inverter I, and an AND gateAND, unlike FIG. 3.

In FIG. 4, the same reference numerals are used to denote the sameelements as in FIG. 3 and thus, a detailed description of the sameelements will be omitted here.

The clock signal generator GEN generates a clock signal “clk” andtransmits the clock signal “clk” to each of the signal delay unit SIGDand the AND gate AND.

The signal delay unit SIGD varies the delay time of the clock signal“clk” in response to a code value of a control code “code” transmittedfrom the controller 4.

The inverter I receives a delayed clock signal “dclk” from the signaldelay unit SIGD, inverts the delayed clock signal “dclk”, and outputs aninverted clock signal “/dclk”.

The AND gate AND performs a logic AND operation on the clock signal“clk” transmitted from the clock signal generator GEN and the clocksignal “/dclk” output from the inverter I and generates a pulse signal“pul” having a pulse width corresponding to the delay time of the signaldelay unit SIGD.

For example, as illustrated in FIG. 5, when the delay time of the signaldelay unit SIGD is “vdt”, the delay time of the clock signal “/dclk”transmitted through the signal delay unit SIGD and the inverter I alsobecomes “vdt”. Thus, the AND gate AND performs a logic AND operation onthe clock signals “clk” and “/dclk” and generates a pulse signal “pul”having a pulse width corresponding to the delay time “vdt” of the signaldelay unit SIGD.

As described above, in the touch sensor of FIG. 4, the pulse signalgenerator 1′, which includes the clock signal generator GEN, the signaldelay unit SIGD, the inverter I, and the AND gate AND, generates thepulse signal “pul” of which pulse width varies with the code value ofthe control code “code”, so that the pulse signal transmitter 2, thepulse signal detector 3, and the controller 4 can operate in the samemanner as described with reference to FIG. 3.

FIG. 6 is a detailed circuit diagram of a touch sensor according to yetanother exemplary embodiment of the present invention.

Referring to FIG. 6, a pulse signal generator 1′ and a pulse signaltransmitter 2 are respectively the same as those of FIG. 4, but a pulsesignal detector 3′ is embodied by a D-flip-flop DFF.

In FIG. 6, the same reference numerals are used to denote the sameelements as in FIG. 4 and thus, a detailed description of the sameelements will be omitted here.

The D-flip-flop DFF receives a clock signal “clk” output from a clocksignal generator GEN as a clock, and receives a pulse signal “pul” asdata. When receiving the pulse signal “pul”, the D-flip-flop DFF issynchronized with a falling edge (or rising edge) of the clock signal“clk”, latches the pulse signal “pul”, and generates a high signal. Whenreceiving no pulse signal “pul”, the D-flip-flop DFF does not latch anysignal and generates a low signal.

Thus, a controller 4 confirms non-contact of a touch object with a touchpad PAD when the D-flip-flop DFF generates the high signal, and confirmscontact of the touch object with the touch pad PAD when the D-flip-flopDFF generates the low signal.

As described above, in the touch sensor of FIG. 6, the D-flip-flop DFFmay vary the level of the output signal depending on whether or not thetouch object contacts the touch pad PAD, so that the controller 4 caneasily confirm the contact or non-contact of the touch object with thetouch pad PAD.

FIG. 7 is a detailed circuit diagram of a signal delay unit SIGDaccording to an exemplary embodiment of the present invention.

Referring to FIG. 7, the signal delay unit SIGD includes a driver D,which is connected to a signal input terminal “clk”, and a plurality ofdelay cells DC1 to DCn, which are connected in series between the driverD and a signal output terminal “dclk”, and each of the delay cells DC1to DCn includes a multiplexer “mux” and inverters I1 and I2.

The driver D buffers a clock signal “clk” and transmits the bufferedsignal to the delay cells DC1 to DCn.

The multiplexers “mux” select the delay cells (e.g., the delay cells DC2to DC0) to perform delay operations in response to code values c0 to cnof a control code “code”, and the multiplexers “mux” and the invertersI1 and I2 included in the selected delay cells DC2 and DC0 delay theclock signal “clk” by a predetermined delay time.

As described above, the signal delay unit SIGD varies the number ofdelay cells to delay the clock signal “clk” according to the code valueof the control code “code” and varies the delay time of the clock signal“clk”, so that the inverter I and an AND gate AND can generate a pulsesignal “pul” having a pulse width corresponding to the delay time of theclock signal “clk”.

Also, the touch sensor according to the present invention may employ avariable resistor of FIG. 8 instead of the resistor R included in thepulse signal transmitter 2, so that the controller 4 can control theresistance of the variable resistor to vary the touch sensitivity of thetouch pad PAD.

FIG. 8 is a circuit diagram of a pulse signal transmitter according toanother exemplary embodiment of the present invention.

Referring to FIG. 8, the pulse signal transmitter includes a variableresistor VR and a touch pad PAD. The variable resistor VR includes aplurality of drivers D0 to Dn, which are respectively connected betweena pulse input terminal “pul” and a plurality of corresponding resistorsR0 to Rn, and the plurality of resistors R0 to Rn are connected inseries to the touch pad PAD.

In this case, a controller (not shown) further provides a control codecode′ for controlling the resistance of the variable resistor VR inaddition to a control code “code” for varying the pulse width of thepulse signal “pul” during a calibration operation.

Thus, the variable resistor VR determines the number of resistors towhich the pulse signal “pul” is transmitted through the drivers D0 toDn, of which operations are controlled in response to code values c0′ tocn′ of the control code code′. In other words, the variable resistor VRvaries the entire resistance according to the code value of the controlcode code′ and also varies an RC time constant with the electrostaticcapacitance of the touch pad PAD.

Thus, charging/discharging characteristics of the touch pad PAD varywith the RC time constant, which is varied by the variable resistor VR,and the touch sensitivity of the touch pad PAD finally depends on thevaried charging/discharging characteristics thereof.

Therefore, the pulse signal transmitter of FIG. 8 may vary the touchsensitivity of the touch pad PAD according to the code value of thecontrol code code′ transmitted from the controller 4.

As described above, the touch sensor according to the present inventionmay vary not only the pulse width of the pulse signal “pul” but also thetouch sensitivity of the touch pad PAD to the touch object according tothe current operating conditions, thus enhancing the preciseness of acalibration operation.

FIG. 9 is a flowchart illustrating a method of operating a touch sensoraccording to an exemplary embodiment of the present invention.

When the touch sensor starts its operation, the pulse signal generator 1generates a pulse signal “pul” having a predetermined pulse width andoutputs the pulse signal “pul” to the pulse signal transmitter 2 in stepS1.

When a touch object is brought into contact with a touch pad PAD, thepulse signal transmitter 2 stops the transmission of the pulse signal“pul” in step S2. When the touch object is out of contact with the touchpad PAD, the pulse signal transmitter 2 transmits the pulse signal “pul”to the pulse signal detector 3 in step S3.

Then, the controller 4 confirms if the pulse signal “pul” is transmittedthrough the pulse signal detector 3 in step S4. As a result, when thepulse signal “pul” is not transmitted, the controller 4 informs a useror an external apparatus that the touch object contacts the touch padPAD in step S5. Thereafter, the controller 4 resets a “non-contactcumulative time” in step S6 and returns to step S1 to perform a newtouch sensing operation.

On the other hand, when it is confirmed in step S4 that the pulse signal“pul” is transmitted, the controller 4 informs the external apparatusthat the touch object is out of contact with the touch pad PAD in stepS7 and confirms if a calibration period comes in step S8.

As a result, when it is confirmed in step S8 that the calibration periodhas not come yet, the controller 4 increases the current “non-contactcumulative time” as much as one unit in step S9 and returns to step S1to perform a new touch sensing operation.

On the other hand, when it is confirmed in step S8 that the calibrationperiod has come, the controller 4 performs a calibration operation suchthat the pulse width of the pulse signal “pul” is adjustable to thecurrent operating conditions in step S10. The calibration of the pulsesignal “pul” in step S10 will be described in more detail with referenceto FIGS. 10 through 12.

When step 10 is finished, the controller 4 resets the current“non-contact cumulative time” and returns to step S1 to perform a newtouch sensing operation using the pulse signal “pul” having a calibratedpulse width.

FIG. 10 is a flowchart illustrating a calibration operation (step S10)of FIG. 9. In FIG. 10, a pulse width appropriate for the currentoperating conditions may be obtained by gradually decreasing the pulsewidth of the pulse signal “pul” from the maximum value.

First, the controller 4 confirms if a “non-contact cumulative time” isequal to or larger than a “non-contact confirmation time” in step S1-1in order to confirm if the current operating conditions are conditionsunder which the calibration of a pulse signal “pul” is normallyperformed (namely, if the touch object is out of contact with the touchpad PAD).

When the “non-contact cumulative time” is less than “the non-contactconfirmation time”, the controller 4 confirms that the touch object isin contact with the touch pad PAD and cancels the calibration operationin step S1-2 and ends the control sequence.

On the other hand, when the “non-contact cumulative time” is equal to orlarger than the “non-contact confirmation time”, the controller 4confirms that the touch object is out of contact with the touch pad fora predetermined duration of time, and fixes the current output state instep S1-3 such that any malfunction does not occur in an externalapparatus due to an output signal of the touch sensor during thecalibration operation.

Thereafter, the controller 4 sets the pulse width of the pulse signal“pul” to the maximum value in step S1-4 and confirms if the pulse signal“pul” is transmitted through the pulse signal transmitter 2 to thecontroller 4 in step S1-5.

When the pulse signal “pul” is transmitted, the pulse width of the pulsesignal “pul” is reduced by one unit in step S1-6 and the controller 4returns to step S1-5. Thus, the pulse width of the pulse signal “pul” isgradually reduced until the pulse signal “pul” is not transmitted.

When the pulse signal “pul” is not transmitted, the controller 4 obtainsthe current pulse width as a critical pulse width in step S1-7 andconfirms if a difference between the current critical pulse width and acritical pulse width obtained in the previous calibration operationexceeds a permitted limit in step S1-8. Here, the permitted limit is avalue that can be determined by a user to confirm if the calibration ofthe pulse signal “pul” is normally performed.

When the difference between the current critical pulse width and thecritical pulse width obtained in the previous calibration operationexceeds the permitted limit, the controller 4 confirms that thecalibration condition is not satisfied and cancels the calibrationoperation in step S1-2 and ends the control sequence.

On the other hand, when the difference between the current criticalpulse width and the critical pulse width obtained in the previouscalibration operation is within the permitted limit, the controller 4confirms that the calibration operation is performed under normalconditions and obtains a calibrated pulse width appropriate for thecurrent operating conditions in step S1-9 by adding a margin pulse widthto the current critical pulse width. Here, the margin pulse width is avalue that can be set by a user based on the touch sensitivity of thetouch pad PAD. Thus, the calibrated pulse width becomes the minimumpulse width that enables the pulse signal detector 3 to detect if thepulse signal “pul” is transmitted under the current operatingconditions.

Thereafter, the controller 4 calibrates the pulse signal “pul” to thecalibrated pulse width in step S1-10, ends the calibration operation,and enters step S11 of FIG. 9.

FIG. 11 is a flowchart illustrating a calibration operation (step S10)of FIG. 9 according to another exemplary embodiment of the presentinvention.

In FIG. 11, a pulse width appropriate for the current operatingconditions may be obtained by gradually decreasing the pulse width ofthe pulse signal “pul” from the sum of the pulse width obtained in theprevious calibration operation and the permitted limit.

In other words, the controller 4 sets the pulse width of the pulsesignal “pul” to the sum of the pulse width obtained in the previouscalibration operation and the permitted limit in step S1-4′, unlike instep S1-4 of FIG. 10. Thereafter, the pulse width of the pulse signal“pul” is gradually reduced in steps S1-5 and S1-6.

As described above, the calibration operation of FIG. 11 aims to obtainthe calibrated pulse width appropriate for the current operatingconditions like the calibration operation of FIG. 10, but the searchablerange of the pulse width is restricted to accelerate the calibrationoperation.

FIG. 12 is a flowchart illustrating a calibration operation (step S10)of FIG. 9 according to yet another exemplary embodiment of the presentinvention

In FIG. 12, a pulse width appropriate for the current operatingconditions may be obtained using a successive approximation method,which is being widely adopted in the analog-to-digital converter (ADC)field.

First, the controller 4 performs the same operations as in steps S1-1 toS1-3 of FIG. 10. Thereafter, the pulse width of the pulse signal “pul”is set to a half “mid” of the maximum value “max”, and a pulse-widthchange unit Δpul is set to an intermediate value between thehalf-maximum value “mid” and the maximum value “max” in step S2-1.

When the pulse signal “pul” is not transmitted in step S2-2, thecontroller 4 increases the pulse width of the pulse signal “pul” bypulse-width change unit Δpul and makes the pulse-width change unit Δpulby half in step S2-3 and returns to step S2-2 again. That is, thecontroller 4 repeats steps S2-2 and S2-3 until the pulse signal “pul” istransmitted to the controller 4 so that the pulse width of the pulsesignal “pul” is gradually increased while increasing.

As a result, when the pulse signal “pul” is finally transmitted in stepS2-2, the controller 4 reduces the pulse width of the pulse signal “pul”by the preset pulse-width change unit Δpul and makes the pulse-widthchange unit Δpul by half in step S2-4 and confirms if the pulse signal“pul” is transmitted in step S2-5. That is, the controller 4 repeatssteps S2-4 and S2-5 until the pulse signal “pul” is not transmitted tothe controller 4 so that the pulse width of the pulse signal “pul” isgradually decreased.

The controller 4 repeats steps S2-2 and S2-5 several times until thepulse width of the pulse signal “pul” is converged to a specific valuein step S2-6, as shown in FIG. 13. Thus, when the pulse width of thepulse signal “pul” is converged to the specific value, the controller 4obtains the specific value as a critical pulse width in step S2-7.

In step S2-6, the pulse width is converged to the specific value byrepeating a process of gradually increasing the pulse width throughsteps S2-2 and S2-3, as shown in FIG. 13, and a process of graduallydecreasing the pulse width through steps S2-4 and S2-5.

The controller 4 confirms if a difference between the current criticalpulse width and the critical pulse width obtained in the previouscalibration operation exceeds a permitted limit in step S2-8. When thedifference between the current critical pulse width and the criticalpulse width obtained in the previous calibration operation exceeds thepermitted limit, the controller 4 confirms that the calibration is notsatisfied and cancels the calibration operation in step S1-2 and endsthe control sequence.

On the other hand, when the difference between the current criticalpulse width and the critical pulse width obtained in the previouscalibration operation is within the permitted limit, the controller 4confirms that the calibration operation is performed under normalconditions and obtains a calibrated pulse width appropriate for thecurrent operating conditions in step S2-6 by adding a margin pulse widthto the current critical pulse width.

Thereafter, the controller 4 calibrates the pulse signal “pul” to thecalibrated pulse width in step S2-7, ends the calibration operation, andenters step S11 of FIG. 9.

As described above, the calibration operation of FIG. 12 aims to obtaina calibrated pulse width appropriate for the current operatingconditions and calibrate the pulse width of the pulse signal “pul”, likethe calibration operation of FIG. 10. It is natural that the decisionstep whether or not the pulse signal is transmitted can be done by asequence way such as, not limited, a train of the same pulse width.

The touch sensor senses only the contact or non-contact of the touchobject and output an output signal but does not output a capacitance ofthe touch object. However, in some cases, the touch sensor not onlysenses the contact or non-contact of the touch object with the touch padPAD but also may have to measure the capacitance in order to measure theintensity of the contact. Also, a portable device including the touchsensor may be affected by frequent environmental changes due to itscharacteristics. Thus, the touch sensor should be configured to lessenthe influence of various noises caused by the environmental changes inorder to inhibit malfunction from occurring in the portable device dueto the noises.

FIG. 14 is a circuit diagram of a capacitance measurement circuitconfigured to sense a contact state according to an exemplary embodimentof the present invention.

The capacitance measurement circuit of FIG. 14 includes a referencesignal generator 110, a variable delay unit 130, a fixed delay unit 130,and a delay time calculator/data generator 140. The reference signalgenerator 110 may be embodied by a clock generation circuit configuredto generate a clock signal having a predetermined period as a referencesignal ref_sig.

The variable delay unit 120 includes a variable delay chain VDC and aresistor R1, which are connected in series between the reference signalgenerator 110 and the delay time calculator/data generator 140. Also,the variable delay unit 120 includes a pad PAD connected between theresistor R1 and the delay time calculator/data generator 140 andconfigured to externally receive a capacitance value. The variable delaychain VDC variably delays and outputs the reference signal ref_sig inresponse to a code value Code fed back and applied by a delay pump 142.The resistor R1 and the pad PAD receive the variably delayed referencesignal ref_sig from the variable delay chain VDC, delay the variablydelayed reference signal ref_sig according to the resistance of theresistor R1 and the capacitance value applied through the pad PAD, andoutput a sensing signal “sen” to the delay time calculator/datagenerator 140.

The fixed delay unit 130 includes a fixed delay chain FDC and a resistorR2, which are connected in series between the reference signal generator110 and the delay time calculator/data generator 140 in parallel to thevariable delay unit 120. The fixed delay chain FDC receives a referencedelay value Nref required for controlling a zero point of a code valueCode applied to the variable delay chain VDC in order to compensate foran offset capacitance of the pad PAD and maximize a measurement range ofthe capacitance value. The fixed delay chain FDC delays the referencesignal ref_sig in response to the reference delay value Nref and outputsthe delayed reference signal ref_sig. The resistor R2 further delays thedelayed reference signal ref_sig output by the fixed delay chain FDC andoutputs a fixed reference signal “ref” to the delay time calculator/datagenerator 140.

Each of the fixed delay chain FDC and the variable delay chain VDC mayinclude a plurality of delay cells like the signal delay unit SIGD ofFIG. 7. Each of the plurality of delay cells may include a singlemultiplexer MUX and two inverters. The fixed delay chain FDC selectsdelay cells configured to delay the reference signal ref_sig in responseto the reference delay value Nref, and the variable delay chain VDCselects delay cells configured to delay the reference signal ref_sig inresponse to the code value Code.

The delay time calculator/data generator 140 includes a phase detector141 and the delay pump 142. The phase detector 141 determines whetherthe phase of the sensing signal “sen” leads or trails that of the fixedreference signal “ref” and outputs a detection signal “det”. The delaypump calculates a capacitance value CV in response to the detectionsignal “det” and ups or downs and outputs the code value Code inresponse to the calculated capacitance value CV.

In the capacitance measurement circuit of FIG. 14, each of the variabledelay chain VDC and the fixed delay chain FDC of the variable delay unit120 and the fixed delay unit 130 directly receives the reference signalref_sig from the reference signal generator 110. Thus, since the pad PADconfigured to externally receive the capacitance is interposed betweenthe variable delay chain VDC configured to receive the fed-backcapacitance value CV and the delay time calculator/data generator 140configured to output the capacitance value CV, the pad PAD is disposedwithin a feedback loop.

Although noises may occur in the capacitance measurement circuit, thereare frequent cases where external noises are applied through the padPAD. Thus, it is most efficient to remove the external noises appliedthrough the pad PAD in order to reduce the noises. Also, in thecapacitance measurement circuit of FIG. 14, the pad PAD through which acapacitance is externally applied is connected to the inside of afeedback loop. When the pad PAD is connected to the inside of thefeedback loop, the noises may be attenuated due to the characteristicsof the feedback loop.

FIG. 15 is a circuit diagram of the capacitance measurement circuithaving the delay time calculator/data generator of FIG. 14. Since areference signal generator 110, a variable delay unit 120, and a fixeddelay unit 130 of FIG. 15 are the same as in FIG. 14, a descriptionthereof will be omitted here.

In a delay time calculator/data generator 140 of FIG. 15, a phasedetector 141 is embodied by a D-flip-flop DFF, and a delay pump 142includes a counter CNT and a subtracter “sub”. The D-flip-flop DFFlatches a sensing signal “sen” in synchronization with one of rising andfalling edges of a fixed reference signal “ref” and outputs the latchedsensing signal “sen”. The D-flip-flop DFF outputs a low-level detectionsignal “det” when the sensing signal “sen” is not delayed with respectto the fixed reference signal “ref”, and outputs a high-level detectionsignal “det” when the sensing signal “sen” is delayed with respect tothe fixed reference signal “ref”. The delay pump 142 increases a codevalue Code when the detection signal “det” drops to a low level, anddecreases the code value Code when the detection signal “det” rises to ahigh level. In other words, the delay pump 142 performs a negativefeedback function and controls such that the phase of the sensing signal“sen” output by the variable delay unit 120 is equal to that of thefixed reference signal “ref” output by the fixed delay unit 130. A delayoffset may occur due to the pad PAD between the fixed delay unit 130 andthe variable delay unit 120. When the delay offset exceeds a controlrange of a variable delay chain VDC, the capacitance measurement circuitof FIG. 15 departs from a normal operation range. In this case, areference delay value Nref of the fixed delay unit 130 compensates foran offset capacitance so that the code value Code of the variable delaychain can be within a variable delay range. Although FIG. 15 shows theD-flip-flop DFF as an example of the phase detector 141, it is naturalthat the phase detector 141 can be embodied by another logic circuit.

The counter CNT is an up/down counter configured to up or down andoutput the capacitance value CV in response to the detection signal“det”. The counter CNT may up or down the capacitance value CV by onebit according to the level of the detection signal “det”. In this case,however, when a large capacitance is applied through the pad PAD, a longtime is taken to measure the capacitance. In order to solve thisproblem, the counter CNT does not up or down the capacitance value CV byone bit but may up or down and output the capacitance value CV inproportion to multipliers of 2, that is, in the order of one bit, twobits, four bits, and eight bits, in response to continuous applicationof high-level or low-level detection signals “det”. Alternatively, thecounter CNT may up and down the capacitance value CV according topredetermined rules in response to the continuous application ofhigh-level or high-level detection signals “det”. Although it isillustrated that the counter CNT receives the detection signal “det” inresponse to the fixed reference signal “ref”, the counter CNT mayreceive the detection signal “det” in response to the reference signalref_sig.

The subtracter “sub” subtracts the capacitance value CV from thereference delay value Nref and outputs the code value Code. Thus, thecapacitance value CV refers to a total capacitance value applied to thepad PAD. When the capacitance value applied to the pad PAD is increased,a feedback loop including the phase detector 141 and the delay pump 142reduces the code value Code by as much as an increment in capacitanceapplied through the pad PAD and reduces the delay amount of the variabledelay chain. Also, when the capacitance value applied to the pad PAD isreduced, the feedback loop increases the code value Code by as much as adecrement in capacitance applied through the pad PAD and increases thedelay amount of the variable delay chain. As a result, the feedback loopcontrols such that the phase of the sensing signal “sen” is equal tothat of the fixed reference signal “ref” applied to phase detector 141,so that the capacitance value CV may correspond to the capacitanceapplied to the pad PAD.

Although it is described here that the reference delay value Nrefapplied to the subtracter “sub” is the same as a signal required tocontrol the fixed delay chain FDC, the subtracter “sub” and the fixeddelay chain FDC may be controlled differently. Also, although it isdescribed that the capacitance measurement circuit of FIGS. 14 and 15includes the fixed delay chain FDC, the fixed delay chain FDC may beomitted.

FIGS. 16 through 18 are diagrams illustrating operations of thecapacitance measurement circuit of FIG. 15. Initially, FIG. 16 showsvariations in a sensing signal “sen” and a detection signal “det” when acapacitance is applied to a pad PAD.

Since a reference signal ref_sig is delayed by a fixed delay unit 130for a fixed delay time and output as a fixed reference signal “ref”, thefixed reference signal ref has the same cycle as the reference signalref_sig. Since a capacitance value CV is 0 during an initial operationof the capacitance measurement circuit, an initial code value Code isequal to a reference delay value Nref, and the time taken for thevariable delay chain VDC to delay the reference signal ref_sig is equalto the time taken for the fixed delay chain FDC to delay the referencesignal ref_sig. Thus, during the initial operation of the capacitancemeasurement circuit, the sensing signal “sen” is delayed with respect tothe fixed reference signal ref due to the capacitance of the pad PAD andoutput to the D-flip-flop DFF.

Since the sensing signal “sen” is delayed with respect to the fixedreference signal “ref”, the sensing signal “sen” is at a high level at afalling edge of the fixed reference signal “ref”, and the detectionsignal “det” is output at a high level. Since the detection signal “det”is at a high level, the counter CNT ups the capacitance value CV andoutputs 1. Since the subtracter “sub” subtracts the capacitance value CVfrom the reference delay value Nref and output the subtraction result,the code value Code is pumped down and output as a reference delay value(Nref)−1.

The variable delay chain VDC reduces the delay time of the referencesignal ref_sig in response to the code value Code and outputs thesensing signal “sen”. As the delay time of the sensing signal “sen”decreases, a difference in delay time between the sensing signal “sen”and the fixed reference signal “ref” also decreases. When the differencein delay time between the sensing signal “sen” and the fixed referencesignal “ref” is gradually reduced until the delay time of the sensingsignal “sen” is equal to or shorter than that of the fixed referencesignal “ref” at a time point t1, the D-flip-flop DFF drops the detectionsignal “det” to a low level.

Thereafter, when a capacitance is externally applied through the pad PADat a time point t2, the sensing signal “sen” is further delayed due tothe applied capacitance, and the D-flip-flop DFF outputs a high-leveldetection signal “det”. When the detection signal “det” is output at ahigh level, the capacitance value CV is gradually increased until thedelay time of the sensing signal “sen” is equal to or shorter than thatof the fixed reference signal “ref” at a time point t3 as describedabove. Thereafter, the capacitance value CV may be oscillated.

FIG. 17 shows a variation in capacitance value CV of a capacitancemeasurement circuit including a counter CNT configured to up or down thecapacitance value CV by one bit, and FIG. 18 shows a variation incapacitance value CV of a capacitance measurement circuit including acounter CNT configured to down or down the capacitance value CVaccording to predetermined rules.

Since the counter CNT performs a count-up or count-down operation by onebit in FIG. 17, when a large capacitance is applied through the pad PAD,it takes a long amount of time for the capacitance value CV to be equalto the applied capacitance. However, since the counter CNT performs thecount-up or count-down operation by one bit, even if a large noise istemporarily applied, the capacitance value CV has only a variation of 1bit. Thus, the influence of the noise on the capacitance value CV isimmaterial.

FIG. 18 illustrates a case where the counter CNT ups or downs thecapacitance value CV according to a rule that when a detection signal“det” is applied at a high level or low level three times in succession,a bit number to be upped or downed is increased. Specifically, when thedetection signal “det” is applied at a high level in succession, thecounter CNT increases a bit number to be upped once every three times.For example, when the detection signal “det” is applied at a high levelor low level in succession, the capacitance value CV is varied by onebit three times and then varied by 2 bits next three times. Also, whenthe detection signal “det” is dropped to a low level, the counter CNTreduces a bit number to be upped or downed and downs the capacitancevalue CV. Thus, even if a large capacitance is applied, the counter CNTmay indicate the capacitance value CV in a short amount of time.Although the capacitance value CV of FIG. 18 may vary within a greaterrange than the capacitance value CV of FIG. 17 in the case of occurrenceof a noise, the influence of the noise on the capacitance value CV issmall because the variation range of the capacitance value CV of FIG. 18is small. In particular, since the pad PAD is disposed within thefeedback loop, the variable delay chain VDC directly receives anoise-free reference signal ref_sig and delays and outputs the referencesignal ref_sig in response to the code value Code, which is obtained inconsideration of a noise and applied through the pad PAD, so that thenoise can be attenuated due to the characteristics of the feedback loop.

Although it is described that the data generator 140 includes aD-flip-flop DFF and an up/down counter CNT and gradually ups or downsthe capacitance value CV, the data generator 140 may count a differencein delay time between the fixed reference signal ref and the sensingsignal “sen” and immediately output the capacitance value CV applied tothe pad PAD.

Furthermore, the delay pump 142 may further include a digital filterconfigured to filter and output a capacitance value CV in order toremove a noise. In the above-described embodiments, since thecapacitance value CV is output within a limited variation range, avariation range of a code value Code is also limited. However, when thecode value Code is varied with a predetermined range or more, it isdetermined that a noise is included. Thus, the delay pump 142 mayfurther include a digital low-pass filter (LPF) or digital band-passfilter (BPF) as the digital filter which receives and filters the codevalue Code from the subtracter “sub” and outputs the variable delaychain VDC. Also, it is natural that the same effect can be obtained byfiltering the capacitance value CV without filtering the code valueCode. The digital filter may be used to control not only noisecharacteristics of the capacitance measurement circuit but also thecharacteristics of the feedback loop. Furthermore, when a difference indelay time between the fixed reference signal ref and the sensing signal“sen” is sufficiently reduced to stabilize the feedback loop, thecapacitance value CV is oscillated by +1/−1 bit. However, theoscillation of the capacitance value CV may be fixed using the digitalfilter. That is, the digital filter may have hysteresis characteristicsto prevent minute oscillation of the capacitance value CV in a steadystate.

Furthermore, the counter CNT and the digital filter may be embodied innot only hardware but also software.

FIG. 19 is a circuit diagram of a capacitance measurement circuitaccording to another exemplary embodiment of the present invention.Since a pulse signal generator 1′ and a pulse signal transmitter 2 ofthe capacitance measurement circuit have the same configurations as inFIG. 4, a detailed description thereof will be omitted.

Also, the pulse signal detector 3 of FIG. 4 adopts only the T-flip-flopTFF. However, as described above, most noises are frequently appliedthrough the pad PAD. Since the T-flip-flop TFF of FIG. 4 is directlyconnected to the pad PAD, when a noise is applied to a pulse signal“pul” through the pad PAD, there is a possibility that the T-flip-flopTFF may be toggled at least once in response to one pulse signal “pul”.The T-flip-flop TFF should be toggled only once in response to one pulsesignal “pul” so that a period determiner 232 of FIG. 19 can accuratelydetermine the periodicity of the pulse signal “pul”. In order to solvethe above-described problem, the T-flip-flop TFF of FIG. 19 may beembodied by the toggle circuit of FIG. 20 to be toggled only once inresponse to one pulse signal “pul”.

In a pulse signal detector 230 of FIG. 19, since the T-flip-flop 231receives the pulse signal “pul” in response to the clock signal “clk”,an output signal of the T-flip-flop 231 is not toggled due to a noise.Also, the pulse signal detector 230 of FIG. 19 may further include theperiod determiner 232 configured to determine whether the output signalof the T-flip-flop 231 is periodically toggled. The period determiner232 determines whether the output signal of the T-flip-flop 231periodically toggles in response to the clock signal “clk”, outputs alow-level detection signal “det” when the output signal of theT-flip-flop 231 periodically toggles, and outputs a high-level detectionsignal “det” when the output signal of the T-flip-flop 231 does notperiodically toggles.

Unlike the touch sensor of FIG. 4 capable of sensing only a contact ornon-contact state, the capacitance measurement circuit of FIG. 19 shouldmeasure a capacitance applied through the pad PAD and output acapacitance value CV, and thus the delay pump 240 includes a counter 241and a digital filter 242 like the capacitance measurement circuit ofFIG. 15. The counter 241 is an up/down counter, which receives thedetection signal “det” in response to the clock signal “clk” and ups ordowns and outputs a counter value Cout by one bit or according topredetermined rules. As described above, the digital filter 242 is usedto control the characteristics of a feedback loop along with the noisecharacteristics of the capacitance measurement circuit. The digitalfilter 242, which has hysteresis characteristic, filters the countervalue Cout and outputs the capacitance value CV to prevent oscillationof the capacitance value CV.

Although it is described that the counter 241 receives the detectionsignal “det” in response to the clock signal “clk”, when the counter 241is a non-synchronous counter, the counter 241 may not receive the clocksignal “clk”. Also, the period determiner 232 may use other signals thanthe clock signal “clk” in order to determine whether the output signalof the T-flip-flop 231 periodically toggles.

In FIG. 19, the variable delay chain VDC variably delays and outputs theclock signal “clk” in response to the capacitance value CV.

FIG. 20 is a circuit diagram of a toggle circuit of the T-flip-flop ofFIG. 19. The toggle circuit includes a single multiplexer Mux 331, anSR-flip-flop SRF 332, and a D-flip-flop DFF 333.

The multiplexer 331 may receive a pulse signal “pul” in response to theoutput signal of the D-flip-flop 333 and applies the pulse signal “pul”to a set terminal S or reset terminal R of the SR-flip-flop 332. Themultiplexer 331 applies the pulse signal “pul” to the set terminal Swhen the detection signal “det” is applied at a low level, and appliesthe pulse signal “pul” to the reset terminal R when the detection signal“det” is applied at a high level.

The SR-flip-flop 332 maintains the level of the previous detectionsignal “det” when no pulse signal is applied from the multiplexer 331,outputs a high-level detection signal “det” to the delay pump 340 when ahigh-level signal is applied to the set terminal S, and outputs alow-level detection signal “det” when a high-level signal is applied tothe reset terminal R.

The D-flip-flop 333 latches the detection signal “det” in response tothe clock signal “clk” applied from the clock signal generator 311 andoutputs the latched signal to the multiplexer 331. The D-flip-flop 333latches the detection signal “det” in response to the clock signal “clk”and determines whether the output signal of the multiplexer 331 isapplied to the set terminal S or reset terminal R of the SR-flip-flop332.

According to the present invention as described above, a touch sensor iscapable of confirming if a touch object is in contact with a touch paddepending on whether a pulse signal is transmitted or not, so that thetouch sensor can perform a touch sensing operation more precisely. Also,the pulse width of the pulse signal is periodically adjusted tooperating conditions, thus preventing the occurrence of a malfunction inthe touch sensor due to changed operating conditions. Thus, theoperating reliability of the touch sensor can be enhanced. Furthermore,in a capacitance measurement circuit configured to detect a contactstate according to exemplary embodiments, a pad through which acapacitance is externally applied is disposed within a feedback loop,and a capacitance value is gradually increased or reduced. As a result,the influence of a noise applied through the pad on the capacitancevalue can be reduced, thus enabling measurement of a precise capacitancevalue.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A capacitance measurement circuit comprising: a pulse signalgenerator for varying a pulse width of a clock signal in response to acapacitance value, and generating a pulse signal; a pulse signaltransmitter including a pad through which a capacitance is externallyreceived, the pulse signal transmitter being configured to or not totransmit the pulse signal in response to the capacitance applied throughthe pad; a pulse signal detector for periodically detecting the pulsesignal applied through the pulse signal transmitter, and outputting adetection signal; a counter for gradually increasing or decreasing andoutputting a counter value according to predetermined rules in responseto the detection signal; and a digital filter for filtering the countervalue and outputting the capacitance value.
 2. The circuit of claim 1,wherein the pulse signal generator comprises: a clock signal generatorfor generating the clock signal; a variable delay chain for variablydelaying the clock signal according to the capacitance value; aninverter for inverting and outputting the output signal of the variabledelay chain; and an AND gate for performing a logic AND on the clocksignal and the output signal of the inverter, and generating the pulsesignal having a pulse width corresponding to a delay time of the clocksignal.
 3. The circuit of claim 1, wherein the pulse signal transmitterfurther comprises a resistor connected between the pulse signalgenerator and the pulse signal detector and configured to inhibittransmission of the pulse signal along with the capacitance appliedthrough the pad.
 4. The circuit of claim 1, wherein the pulse signaldetector comprises: a T-flip-flop for detecting the pulse signal inresponse to the clock signal, and generating an output signal thattoggles in response to the pulse signal; and a period determiner fordetermining whether the output signal of the T-flip-flop periodicallytoggles, and outputting the detection signal.
 5. The circuit of claim 4,wherein the T-flip-flop comprises: an SR-flip-flop for outputting thedetection signal in response to the pulse signal applied to one of a setterminal and a reset terminal; a D-flip-flop for latching and outputtingthe detection signal in response to the clock signal; and a multiplexerfor selecting one of the set terminal and the reset terminal in responseto the output signal of the D-flip-flop, and transmitting the pulsesignal to the selected terminal.
 6. The circuit of claim 1, wherein thecounter gradually increases or decreases the capacitance value inresponse to the detection signal by a predetermined unit and outputs thecapacitance value.
 7. The circuit of claim 1, wherein when the detectionsignal is continuously applied at a high level or low level, the countervaries the capacitance value by a variation unit of the capacitancevalue and gradually increases or decreases and outputs the capacitancevalue.
 8. The circuit of claim 1, wherein the digital filter is an LPFor BPF configured to receive and stabilize the counter value, remove anoise, and output the capacitance value.
 9. The circuit of claim 1,wherein the counter and the digital filter are embodied in software.